of bugs easier (MDEV-8919, MDEV-10304, MDEV-10305, MDEV-10307)

- Adding Item::push_note_converted_to_negative_complement() and
  Item::push_note_converted_to_positive_complement()
- Adding virtual methods Item::val_int_signed_typecast() and
  Item::val_int_unsigned_typecast()
- Moving COLUMN_GET() related code from
  Item_func_signed::val_int() and Item_func_unsigned::val_int() to
  Item_dyncol_get::val_int_signed_typecast() and
  Item_dyncol_get::val_int_unsigned_typecast()
- Moving Item_func_signed::val_int_from_str() to Item::val_int_from_str()
  and changing it to get the value from "this" instead of args[0].

The patch does not change behaviour. It's only to simplify fixing of the
mentioned bugs. It will also simplify switching the CAST related code to
use the type handler infrastructure easier (soon).